In recent years, apparatuses for compressing and encoding an image by employing an encoding scheme in which image information is treated as digital data and, at this time, for the purpose of high-efficiency information transmission and accumulation, compression is performed through orthogonal transform such as discrete cosine transform and motion compensation by using redundancy unique to the image information have been widely used. The encoding scheme includes, for example, an MPEG (Moving Picture Experts Group) or the like.
Particularly, the MPEG-2 (ISO/IEC 13818-2) scheme is defined as a general-purpose image encoding scheme and is a standard covering both of interlaced scanning images and sequential scanning images and covering standard resolution images and high-accuracy images. For example, the MPEG-2 scheme is widely used for a wide range of applications of professional uses and consumer uses. By using the MPEG2 compression scheme, for example, a code amount (bit rate) of 4 to 8 Mbps is allocated to an interlaced scanning image having a standard resolution of 720×480 pixels. In addition, by using the MPEG2 compression scheme, for example, a code amount (bit rate) of 18 to 22 Mbps is allocated to an interlaced scanning image having a high resolution of 1920×1088 pixels. Therefore, a high compression rate and a good image quality may be implemented.
The MPEG-2 is mainly applied to high image quality encoding which is suitable for broadcasting, but it does not correspond to an encoding scheme having a code amount (bit rate) lower than that of the MPEG1, that is, an encoding scheme having a higher compression rate. With the spread of mobile phones, needs for the encoding scheme are expected to be increased, and accordingly, the MPEG-4 encoding scheme is standardized. With respect to the image encoding scheme, the ISO/IEC 14496-2 standard was approved as an international standard in December, 1998.
In addition, in recent years, for the purpose of image encoding for TV conference, standardization called H.26L (ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Expert Group)) has been promoted. It is known that, in comparison with the encoding schemes such as the MPEG-2 or the MPEG-4 in the related art, in the H.26L, although a large calculation amount is needed for encoding and decoding, a higher encoding efficiency is implemented. In addition, at present, as a part of activities of the MPEG-4, standardization which is based on the H.26L and incorporates functions which are not supported in the H.26L to implement a higher encoding efficiency is performed as Joint Model of Enhanced-Compression Video Coding.
As the schedule of the standardization, the standard was approved as an international standard on the basis named H.264 and MPEG-4 Part 10 (Advanced Video Coding, hereinafter referred to as AVC)) in March, 2003.
In addition, as extension of the H.264/AVC, standardization of Fidelity Range Extension (FRExt) including RGB, encoding tools necessary for business such as 4:2:2 or 4:4:4, 8×8 DCT defined by the MPEG-2, and quantization matrices was completed in February, 2005. Accordingly, the H.264/AVC became an encoding scheme capable of representing film noise included in a movie with a good quality. Therefore, the H.264/AVC has been used for a wide range of applications such as Blu-Ray disc (trade mark).
However, recently, needs for high compression rate encoding, for example, a need to compress images of about 4000×2000 pixels which is four times of a high-vision image or a need to distribute a high-vision image in a limited-transmission-rate environment such as the Internet have been further increased. Therefore, in the VCEG under the ITU-T, improvement of an encoding efficiency continues to be studied.
Therefore, at present, for the purpose of further improvement of the encoding efficiency in comparison with the AVC, standardization of an encoding scheme called high efficiency video coding (HEVC) has been promoted by the joint collaboration team-video coding (JCTVC) as a joint standardization body of the ITU-T and the ISO/IEC. With respect to the HEVC standard, Committee Draft as a first draft specification was issued in February, 2012 (for example, refer to Non-Patent Document 1).
However, in an intra 8×8 prediction mode of the AVC encoding scheme, a [121]/4 filtering process is performed on neighboring pixels of a current block which is a processing target. In the HEVC, on/off of the filtering process is determined according to a block size and a prediction mode.
In addition, in the HEVC, for the purpose of reducing block distortion of the case where the prediction mode is a DC mode, a horizontal mode, or a vertical mode, a boundary value smoothing process is defined.
In addition, in order to reduce a phenomenon that a contour is seen at a flat portion of an image, a contour noise countermeasure process was proposed (for example, refer to Non-Patent Document 2).
In the method disclosed in Non-Patent Document 2, features of the neighboring pixels of the current block which is a processing target are identified by performing a threshold value determining process. In the case where a result of the threshold value determining process is true, namely, in the case where the neighboring pixels have predetermined features, instead of the above-described [121]/4 filtering process, a bi-linear interpolation process (referred to as a bi-linear filtering process) is performed.